That is roughly thirty people across, so, as they march quickly forward (faster than usual this year because most of them are hurrying [albeit joyfully and devoutly] to catch their buses to leave before the brunt of the blizzard); that's at least thirty people per second! Multiply that by the total time of the March. In half hour that would be, conservatively, 54,000. 100,000 in an hour, 150,000 in an hour and a half.
Or you could try calculating by distance. Google maps gives me distance over 7,500 feet (it says 1.6 miles). At that rate, giving each row of 30 people five feet on the street going forward would make 43,000. But it looks like there are still people packed in the back of the procession and in the front so that number could be double, triple, quadruple, etc., depending on how many times that scene repeats.
A third way to calculate would be to zoom in on the picture below and actually count the people, though that picture would also only be the people at that one moment. You would have to have a picture for each successive group of the procession as the demonstration progressed. If one picture is 43,000 and it is repeated three or four times in the afternoon (e.g. at 1:30, 2:00, 2:30, 3:00), that would mean between 100,000 and 150,000.
The EWTN live video footage above gives a good idea of the size of the crowd. For the sake of getting an accurate count they have a few stationary cameras along the March route to show the massive number of people going through. Go to the 4:35 minute to get a good picture of the quickly passing, apparently unending, multitude and that goes on for thirty minutes on the video! Certainly by the 4:46 minute the front of the March has reached the Supreme Court destination and from at least that moment the entire avenue is filled from the Washington Monument to the Supreme Court.
Why doesn't someone just count the people? Maybe I'll do it next year. I'll stand at the high point of the hill and watch the whole procession from start to finish with a counter in my hand to count each row of 30 people crossing my line, and perhaps arrange it with EWTN to finally get an official accurate count!